It is known to dissolve the cellulose in systems constituted by organic solvents and other substances which may react with the cellulose, and systems of this kind are described in the literature, however they have a purely academic interest since they are not adapted, as far as is now known, for industrial operations.
The system constituted by paraformaldehyde and formaldehyde or DMSO has been recently suggested for this purpose. However it is not easy to obtain, especially in an industrial process, regenerated cellulose formed bodies from such solutions. The most interesting bodies are the filaments of regenerated cellulose or also the films, which must be prepared through coagulation, cellulose regeneration and drawing stages. In U.S. Pat. No. 4,022,631 a system is described and claimed for coagulating and regenerating the cellulose from said solutions, which consists in using as coagulating bath an aqueous solution which is alkaline because of the presence of ammonium or amino compounds or alkaline sulphites or thiosulphates, in particular aqueous ammonia.
In French Patent Application No. 76 22031 it is recalled that it has been attempted to coagulate cellulose solutions by extruding them from a syringe into a methanol bath, but that the product thus coagulated does not withstand the spinning tensions and does not lead to fibres having acceptable properties, so that this is not a method that can be used industrially.
In copending Italian Patent Application No. 20540 A/78 of the same Applicant, a process is described for obtaining solutions of cellulose derivatives in organic solvents different from DMSO, such as e.g. dimethylformamide (DMF), by dissolving the cellulose in a system constituted by paraformaldehyde or formaldehyde and DMSO and thereafter isolating the cellulose derivative thus formed and dissolving it in the desired organic solvent.
In copending Italian Patent Application No. 21844 A/78 of the same Applicant, which is an addition to the aforementioned one, an improvement is described which consists in preparing the cellulose derivative by heating cellulose and paraformaldehyde, with amounts of DMSO which are lower than those required for the preparation of a solution in DMSO.
The solutions prepared according to said patents may be coagulated and the cellulose may be regenerated, e.g., with baths which are alkaline due to the presence of ammonia.
In copending Italian Patent Application No. 21922 A/78 of the same Applicant another process is described for forming solutions of cellulose derivatives in organic solvents, which consists in contacting the cellulose with a strong excess of paraformaldehyde and with a solvent selected from DMF, dimethylacetamide and N-methylpyrrolidone, at an elevated temperature. These solutions too may be coagulated and the cellulose may be regenerated in the known manner.
Said patents describe various methods for bringing the cellulose in solution in organic solvents with the formation of cellulose derivatives, the exact chemical structure whereof is not known but which are generally designated as "methylol derivatives" and will be so designated herein, without the Applicant being thereby bound to any interpretation with regard to their structure.